1. Field of the Invention
The invention relates to a transmitting apparatus and a sound sensor for obtaining information such as the presence/absence of a detection-target object with an acoustic wave, or a distance therefrom to the detection-target object, and further relates to an autonomous traveling vehicle equipped with such a sound sensor.
2. Description of Related Art
Conventionally, there has been known an ultrasonic sensor that transmits an ultrasonic wave to a detection-target object and, also, receives a reflected wave from the detection-target object to thereby measure a distance therefrom to the detection-target object on the basis of an elapsed time from a transmission start time point to a reception start time point.
As a kind of such an ultrasonic sensor, there has been known an ultrasonic sensor comprising a transmitting apparatus including: a transmitter which includes a piezoelectric element and transmits an ultrasonic wave; a driving circuit which drives the piezoelectric element; and an oscillation circuit which inputs an oscillation signal in a predetermined frequency band to the driving circuit, and a receiving apparatus including: a receiver which includes a piezoelectric element and receives an ultrasonic wave; a sensing circuit which senses an ultrasonic wave in a predetermined frequency band on the basis of an electric signal generated as determined by a vibration of the piezoelectric element caused by the action of the ultrasonic wave; and a signal processing unit which performs processing for obtaining information on a detection-target object on the basis of the electric signal (see, for example, Japanese Patent Application Laid-Open No. 10-290495 (1998)).
The transmitter of the ultrasonic sensor described above has a case in the shape of a hollow cylinder with a bottom working as a vibration plate, to the inner surface of which a piezoelectric element is adhered. The driving circuit of the same vibrates the piezoelectric element at a predetermined resonance frequency band by applying a voltage to the piezoelectric element on the basis of an oscillation signal from the oscillation circuit. The resonance frequency band by which the piezoelectric element is vibrated is regarded as a resonance frequency at which a vibration system including the piezoelectric element resonates the most strongly and in which an ultrasonic wave is transmitted with a large amplitude of the vibration plate in resonance.
The ultrasonic wave transmitted from the transmitter is reflected by a detection-target object and received by the receiver. The ultrasonic wave received by the receiver is converted to an electric signal and the electric signal is signal-processed at the signal processing unit to thereby determine a distance therefrom to the detection-target object.
A conventional ultrasonic sensor as mentioned above, as shown in timing charts of FIG. 1A to FIG. 1C, has a slow rise of acoustic pressure of a transmitted ultrasonic wave until a vibration system including the piezoelectric element resonates in a stable state from the time when a voltage is applied to the piezoelectric element, for which transition a time T1 elapsed.
To be concrete, the time T1 elapsed until the vibration system including the piezoelectric element resonates in a stable state, as shown in an output waveform of the transmitter of FIG. 1B, from the time when a voltage is applied to the transmitter, as shown in an output waveform of the driving circuit of FIG. 1A.
In a case where an ultrasonic wave with such a slow rise of acoustic pressure is received by the receiver, a time delay arises until the time when an acoustic pressure of the ultrasonic wave rises to a predetermined acoustic pressure from the time when the ultrasonic wave actually reaches the receiver. Since a configuration is such that determination on whether or not an ultrasonic wave has been received is performed by whether or not an electric signal obtained by converting an acoustic pressure of the received ultrasonic wave exceeds a predetermined threshold value, a measurement error arises as a time T2, shown in an input waveform of the receiver on FIG. 1C, elapsed until the input waveform exceeds a predetermined threshold value from the time when the ultrasonic wave starts to be received. The conventional sound sensor has had an unsolved problem that a measurement error, to be concrete, the time T2 shown in FIG. 1C, indispensably arises due to such a slow rise of acoustic pressure of an ultrasonic wave.